1. Field of the Invention
The present invention relates to a coated layer type resistor device. The resistor device according to the present invention is used for, for example, a bridge circuit in electronic apparatuses.
2. Description of the Prior Art
In a prior art process for manufacturing a coated layer type resistor device consisting of a first resistor element and a second resistor element, conductor electrodes for the first resistor element and conductor electrodes for the second resistor element are formed on an insulator substrate made of alumina ceramics by a screen printing process. A resistor layer for the first resistor element is then formed on the substrate between the conductor electrodes for the first resistor element and portions of the conductor electrodes for the first resistor element by a screen printing process, and a resistor layer for the second resistor element is formed on the substrate between the conductor electrodes for the second resistor element and portions of the conductor electrodes for the second resistor element by a screen printing process. Then a heat treatment is carried out to establish electrical connection between the resistor layer for the first resistor element and the conductor electrodes for the first resistor element, and between the resistor layer for the second resistor element and the conductor electrodes for the second resistor element.
During this heat treatment process, some of the constituents of the end conductor electrodes, such as silver or platinum, may be diffused into the resistor layers to form boundary portions in the resistor layers adjacent to the end conductor electrodes. Thus, in each of the boundary portions in the resistor layers, the electrical property of the resistor layer has been changed. It is known that the temperature coefficient of the resistance (TCR) of the first resistor element 1 is greatly affected by the TCR of such boundary portions of the resistor layers.
Under this condition, when the ratio R.sub.2 /R.sub.1 of the resistances of the second and the first resistor elements is selected to be greater than a predetermined ratio, the relationship between the length l.sub.2 of the resistor layer between the conductor electrodes for the second resistor element and the length l.sub.1 of the resistor layer between the conductor electrodes for the first resistor element becomes l.sub.1 &lt;l.sub.2.
Under such a relationship, wherein l.sub.1 &lt;l.sub.2 between the first and second resistor elements, the influence of the boundary portions in the resistor layer for the first resistor element on the TCR of the first resistor element is different from the influence of the boundary portions in the resistor layer for the second resistor element on the TCR of the second resistor element.
Thus, the TCR of the first resistor element becomes different from the TCR of the second resistor element, and the ratio R.sub.2 /R.sub.1 is changed as the circumferential temperature is changed. Accordingly, it is difficult to realize the ratio R.sub.2 /R.sub.1 with a high precision. This constitutes a problem in the prior art.
Even if the lengths of the resistor layers of the first and the second resistor elements are made equal, in order to equalize the TCR's of the first and the second resistor elements, the width d.sub.1 of the resistor layer for the first resistor element must be greater than the width d.sub.2 of the resistor layer for the second resistor element, for maintaining the ratio R.sub.2 /R.sub.1 at a value greater than a predetermined ratio. This requirement makes it necessary to increase the size of the resistor layer for the first resistor element, and such an increase in the size of the resistor layer causes another problem in the structure and the manufacturing process of a resistor device.